To focus light in or through a scattering medium using wavefront shaping techniques, one need a way to probe the intensity or the field at the target position. To avoid having to insert a probe in the medium, Xu et al. proposed in 2011 the use of an ultrasonic focused beam to select a target area by photo-acoustic effect [X. Xu, H. Liu and L.V. Wang, Nat. Photon., 5, 154, (2011)]. This technique allows to focus light on a spot of the size of the ultrasound focused beam, which is typically at least one order of magnitude larger than the optical wavelength. In this new study, B. Judkewitz and co-authors used an innovative method to be able to focus light on a much smaller scale.

To focus light on a scattering region using an ultrasound focused beam the procedure is the following. The medium is optically excited on one side. The light spread into the medium while an ultrasonic beam is focused on a defined region of the sample. Photons that pass through this region are frequency shifted by acousto-optic effect. One record only the output light that has been ultrasonically excited. The wavefront is phase conjugated, sent back onto the medium and the light focuses on the area of the initial acoustic excitation (figure 1.a.)

Knowing that the ultrasound focal spot is much bigger that the optical wavelength, there is many ways to focus light on that area. By using random illuminations, the authors can create a database of signals that focus on a given ultrasound focal spot. Because the ultrasound spot has a Gaussian shape, the components of light in the database that passes through the center or the edge of this region do not have the same statistics. The authors then record databases of signals for four overlapping ultrasound focal spots. By sorting the signals using their relative variances in the four databases, they are able to find what signal focuses where on spots of 5 microns, which is 6 times smaller than the acoustic focused beams used (figure 1.b.).